The field of the invention relates to engine speed control systems. More specifically, the field relates to smoothly decelerating an engine to an idle speed.
Engine idle speed control systems are known which control idle speed by controlling the amount of air inducted through a bypass passageway arranged in parallel with the main throttle plate. It is also known to use the bypass air control to smooth engine deceleration from an engine speed significantly above idle towards an idle speed in a predetermined, preprogrammed manner. An example of such a control system is shown in U.S. Pat. No. 5,261,368.
The inventors herein have recognized numerous problems with the above approach. They have found that by decelerating the engine in a preprogrammed manner, speed undershoot or speed overshoot from a desired vehicle speed may result. For example, the required air flow to bring the engine speed down smoothly to the idle region is different for different load and operational conditions such as temperature, speed ranges, road terrain, and it may also vary with vehicle aging. In addition, the rate of deceleration may be unsatisfactory over all load and speed conditions.